fix query result for explain anaylyze (#16265)

问题原因：
explain 与 explain analyze走得不同的逻辑。
提取explain anaylze column defs 的逻辑不对。

修改方案：
为explain anaylze设置正确的column defs。

Approved by: @qingxinhome

pkg/frontend/mysql_cmd_executor.go

@@ -1823,7 +1823,7 @@ func handleEmptyStmt(ses FeSession, execCtx *ExecCtx, stmt *tree.EmptyStmt) erro
 	return err
 }
 
-func GetExplainColumns(ctx context.Context, explainColName string) ([]interface{}, error) {
+func GetExplainColumns(ctx context.Context, explainColName string) ([]*plan2.ColDef, []interface{}, error) {
 	cols := []*plan2.ColDef{
 		{Typ: plan2.Type{Id: int32(types.T_varchar)}, Name: explainColName},
 	}
@@ -1834,11 +1834,11 @@ func GetExplainColumns(ctx context.Context, explainColName string) ([]interface{
 		c.SetName(col.Name)
 		err = convertEngineTypeToMysqlType(ctx, types.T(col.Typ.Id), c)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		columns[i] = c
 	}
-	return columns, err
+	return cols, columns, err
 }
 
 func getExplainOption(reqCtx context.Context, options []tree.OptionElem) (*explain.ExplainOptions, error) {
@@ -2836,6 +2836,8 @@ func doComQuery(ses *Session, execCtx *ExecCtx, input *UserInput) (retErr error)
 
 	defer func() {
 		ses.SetMysqlResultSet(nil)
+		ses.rs = nil
+		ses.p = nil
 	}()
 
 	canCache := true

pkg/frontend/result_row_stmt.go

@@ -29,6 +29,7 @@ import (
 // executeResultRowStmt run the statemet that responses result rows
 func executeResultRowStmt(ses *Session, execCtx *ExecCtx) (err error) {
 	var columns []interface{}
+	var colDefs []*plan2.ColDef
 	ses.EnterFPrint(63)
 	defer ses.ExitFPrint(63)
 	switch statement := execCtx.stmt.(type) {
@@ -73,14 +74,16 @@ func executeResultRowStmt(ses *Session, execCtx *ExecCtx) (err error) {
 
 	case *tree.ExplainAnalyze:
 		explainColName := "QUERY PLAN"
-		columns, err = GetExplainColumns(execCtx.reqCtx, explainColName)
+		colDefs, columns, err = GetExplainColumns(execCtx.reqCtx, explainColName)
 		if err != nil {
 			ses.Error(execCtx.reqCtx,
 				"Failed to get columns from ExplainColumns handler",
 				zap.Error(err))
 			return
 		}
 
+		ses.rs = &plan.ResultColDef{ResultCols: colDefs}
+
 		ses.EnterFPrint(66)
 		defer ses.ExitFPrint(66)
 		err = respColumnDefsWithoutFlush(ses, execCtx, columns)